1. Field of the Invention
The present invention relates to optical modulators and switches, and in particular, fiber-based optical modulators and switches.
2. Description of the Related Art
Presently, only a few kinds of phase modulators operating in the MHz frequency range are commercially available. For example, the electro-optic lithium niobate modulator can be designed to operate up to many hundreds of GHz. Lithium niobate modulators are relatively compact (a few cm in length), and require only a few volts when they are constructed in waveguide form and a few hundreds of volts when they are in bulk-optic form. On the other hand, they exhibit a fairly high internal loss of at least 1 dB, as well as coupling losses of at least 0.5 dB per port. Thus, the fiber-to-fiber loss of a pigtailed lithium niobate modulator is at least 2 dB, and in many products it is considerably higher. Also, the cost of these devices is high, typically a few thousand dollars. Furthermore, in the case of bulk-optic lithium niobate modulators, the voltage required is on the order of a few hundred volts when operating at multi-megahertz frequencies. This voltage requirement is met by a resonant electronic circuit that boosts a low input voltage signal of a few volts, but such a circuit generally has a limited bandwidth of typically around 1 MHz, so that the modulator operates over a narrow frequency range.
Another kind of high-frequency phase modulator is a piezoelectric (PZT) ring modulator. In this device, a fiber that is typically a few meters in length is wound around a PZT ring. When an AC voltage is applied to the ring, the ring expands and contracts periodically, thereby stretching the fiber, which then modulates the phase of an optical signal traveling through the fiber. While this type of modulator required only a few volts, it produces a useful phase shift (typically around xcfx80) at only at a few discrete frequencies corresponding to the mechanical resonant frequencies of the ring. Thus, the bandwidth of this device is also limited.
A third type of phase modulator is the acousto-optic (A/O) fiber modulator, in which a fiber is coupled mechanically to a PZT modulator, which compresses it periodically. (See, for example, I. Abdulhalim, and C. N. Pannell, xe2x80x9cPhotoelastic in-fiber birefringence modulator operating at the fundamental transverse acoustic resonance,xe2x80x9d IEEE Photon. Techno. Lett. vol. 5, no. 10, pp. 1197-1199, October 1993.) This type of modulator is also driven by a resonant electronic circuit, so that its bandwidth is generally limited to on the order of 1 MHz. An A/O modulator may require 0.7 W of input power to produce a phase modulation of xcfx80/2. Also, A/O fiber modulators in which the fiber is coated with a thin PZT film have been demonstrated at Stanford University. While A/O fiber modulators work well, they only operate at discrete resonant frequencies and require a fairly high input electrical power.
For all of these modulators, a signal of one polarization traveling through the device will undergo a phase modulation that is significantly different from a signal having the orthogonal polarization. This polarization dependence is highly undesirable in many applications, because the polarization of an input signal is generally not constant but rather drifts unpredictably over time.
Although there exists a variety of optical fiber components such as filters, amplifiers, couplers, and lasers, all-fiber optical modulators and switches with suitable characteristics are presently not readily available. Such devices would be useful in fiber sensors, fiber sensor arrays, optical communication systems, and fiber and waveguide devices such as lasers.
One aspect of the present invention is an apparatus for modulating the phase of an optical signal. The apparatus comprises an optical medium for propagating the optical signal, as well as first and second electrodes proximate to the optical medium. The first and second electrodes have an AC voltage imposed therebetween which causes strains in the optical medium which, via the electrostrictive effect, produce variations in the index of refraction of the optical medium. In one preferred embodiment of the invention, the optical medium is unpoled, and the electrodes may apply a DC voltage in addition to the AC voltage. The phase of the optical signal may be modulated such that polarization components of the optical signal parallel to and orthogonal to the electric field experience an equal phase shift. The apparatus is advantageously incorporated into an interferometer to form a device that modulates the amplitude of the optical signal. Alternatively, the apparatus is incorporated into an interferometer to form an optical switching device.
Another aspect of the present invention is an apparatus for modulating the phase of an optical signal, in which the apparatus comprises a poled optical medium for propagating the optical signal. The poled optical medium has an internal DC field. At least one electrode is positioned in proximity to the medium. The electrode has an AC voltage applied thereto to induce an AC electric field within the medium to produce variations in the index of refraction of the optical medium through the electrostrictive effect. In one preferred embodiment, the phase of the optical signal is modulated such that polarization components of the optical signal parallel to and orthogonal to the electric field experience an equal phase shift. The apparatus is advantageously incorporated into an interferometer to form a device that modulates the amplitude of the optical signal. Alternatively, the apparatus is advantageously incorporated into an interferometer to form an optical switching device.
Another aspect of the current invention is a method of modulating the phase of an optical signal by providing an optical medium, applying an AC voltage to produce an electric field within the optical medium, producing variations in the index of refraction of the optical medium through the electrostrictive effect by causing strains in the optical medium, and passing an optical signal through the optical medium to modulate the phase of the optical signal. In a preferred embodiment of this method, the AC voltage is applied to first and second electrodes, in which the electrodes are in proximity to the optical medium. In another preferred embodiment of this method, the AC voltage is applied at a frequency such that polarization components of the optical signal parallel to and orthogonal to the electric field experience an equal phase shift as the optical signal passes through the optical medium. Output from the optical medium may be directed into an interferometer to modulate the amplitude of the optical signal, or to switch the optical signal from a first output port of the interferometer to a second output port of the interferometer.